This invention relates to high-frequency electron tube power oscillator comprising an adjustable impedance element connected in series with the cathode of the electron tube.
A high-frequency electron tube power oscillator of this type is known from Dutch Patent Specification 148202.
Generally, it is advantageous to be able to control the power supplied by such high-frequency electron tube oscillators.
A first method of realizing this control is to make the D.C. anode voltage of the electron tube variable. This can be achieved by control of the firing angle of the applied mains voltage by means of thyrisors, before transforming this voltage to the desired high voltage.
Power control by control of the firing angle causes rather substantial high-frequency noise-voltage on the mains. A second disadvantage of this type of power control is the fact that the high voltage is equal to zero for a part of the period of the mains voltage. Consequently, a large and costly high voltage smoothing filter will be necessary in many cases.
A second method of controlling the supplied power is described in the aforementioned Patent Specification. In that document the power supplied is controlled by means of an adjustable impedance element connected in series with the cathode of the electron tube.
By increasing the impedance value of the impedance element, the D.C. voltage across the impedance element increases. As a result of this increase the voltage difference between the cathode and control grid becomes more negative. Consequently, the D.C anode current of the electron tube and thus the supplied power decreases.
A disadvantage of this prior art high-frequency power oscillator is the large power dissipation occuring in the adjustable impedance element. This power may be about 10% of the power to be controlled. This is especially detrimental for large powers (for example, powers of several hundred kilowatts), more particularly if the impedance element is to be arranged in the form of an adjustable semiconductor element.